Field of the Invention
The present invention relates to aluminum alloys (also known as Al alloys), especially to 7xxx series aluminum alloys (Al—Zn—Mg—Cu based aluminum alloys) as designated by the International Aluminum Association. In particular, the present invention relates to large thickness (e.g., from 30˜360 mm) products made from 7xxx series aluminum alloys. Although the present invention is directed to large thickness forged product shapes and rolled plate product forms in the most cases, it can also be used for extrusions and cast products having a large thickness entirely or locally.
Description of the Prior Art
In the modern aerospace manufacturing, with the increasing requirements of integrated flight performance, payload fuel consumption, service life, and reliability of aircrafts, large integral aluminum alloy structural components are widely used in aircrafts more and more. For instance, in the design and manufacturing of junctions of the wing and the body of an aircraft, using an integral wing-body built-up component of wing and body of an aircraft made from a large-scale aluminum alloy product having a homogeneous composition and prepared by a numerical control milling process, instead of a conventional combination built-up component assembled with a plurality of separate aluminum alloy parts having various composition, not only can reduce substantially the weight of components and increase the reliability during service life, but also can reduce substantially the procedures of assembling components and decrease the comprehensive costs for manufacturing an aircraft.
However, such advanced design and manufacturing method leads to a highly harsh requirement for the comprehensive performance of relevant aluminum alloy products.
As well known in the field of aircraft manufacturing, with regard to materials of making the face surface of the wing or the built-up structure of wing and body of an aircraft, it is commonly desirable that they possess an optimal compressive yield strength, as well an acceptable damage tolerance property; whereas with regard to materials of making the back surface of the wing or the built-up structure of wing and body of an aircraft, it is commonly desirable that they possess an optimal damage tolerance property, as well as an acceptable tensile yield strength. In a conventional combinational structure, the aforesaid object can be achieved by assembling a plurality of aluminum alloy parts having various compositions. As an example, when designing and selecting the materials of making the face surface, it is preferable to use aluminum alloys having a higher level of compressive yield strength and an acceptable damage tolerance property, such as, 7150, 7055, 7449 alloys or the like; and when designing and selecting the materials of making the back surface, it is preferable to use aluminum alloys having an acceptable tensile yield strength and an optimal damage tolerance property, e.g., 2324, 2524 alloys, or the like. However, (1) if the aforesaid structure is designed as an integral structure, the single alloy product as used should possess both the optimal tensile and compressive yield strengths and the optimal damage tolerance property, namely, possess the so-called “optimal combination of properties”; and (3) some integral components tend to have a greater local thickness, whereby causing that the aluminum alloy products for making these integral components should have a great thickness of, e.g., 30 mm or greater, or even up to 360 mm. For ensuring the property consistency of various sites of the integral component, it is desired that various sites within the aluminum alloy products should possess highly homogeneous properties.
Through testing the over-all properties, it has been found that some conventional high-strength and high-toughness aluminum alloys broadly used in the field of aircraft manufacture cannot satisfy the before identified requirements. For instance, 7050, 7150 alloys, etc., are well known in the field as the aluminum alloys having a good balance of various properties. Products made from these alloys having a thickness of 20˜80 mm can exhibit good over-all properties in both the surface and the core, as well as acceptable differences between the surface and the core; but product of these alloys having a thickness of up to 150 mm represent a yield strength of the core at least 10% lower than the yield strength of the surface and remarkable differences of elongation and fracture toughness, even though they still can maintain the good over-all properties in the surface. Moreover, 7055, 7449 alloys, etc., are well known in the art as wrought high-strength aluminum alloys. Products made from these alloys having a thickness of 20˜60 mm can represent desirably a high strength in both the surface and the core, as well as acceptable differences between the surface and the core; but product of these alloys having a thickness of up to 100 mm represent a yield strength, elongation, fracture toughness, threshold of fatigue fracture, corrosive nature of the core at least 10%˜25% lower than those of the surface, even though they still can maintain substantially a high strength and other over-all properties in the surface. A well-established principle is that the designers select materials on the basis of the minimal guaranteed properties of the materials during the designing of the structure of aircraft. According to this principle, when conventional alloys including 7050, 7150, 7055, 7449, or the like are processed to products having a smaller thickness of e.g., 80 mm or below, there is a good comprehensive performance consistency between the surface and the core, and the minimal ensurable property (typically, the core properties) can satisfy the requirements of manufacturing some structural components having a higher load-bearing; however, if these alloys are processed to large thickness products, the core properties deteriorate remarkably, and the minimal ensurable properties of the products have become incapable of satisfying the requirements of manufacturing some structural components having a higher load-bearing. Furthermore, products made from 7xxx series aluminum alloys represents too large differences between the surface and the core, whereby resulting in some unexpected problems during subsequent processing, such as, a relatively high residual internal stress, as well as the hardness of establishment and operation of subsequent milling process. It is undesired for the designers of aircraft.
A great number of research results indicate that the property differences between the surface and the core of large thickness products made from 7xxx series aluminum alloys are primarily due to the quenching process after the solution heat treatment of the alloys. FIG. 1 shows the curve of quenching of large thickness products made from 7xxx series aluminum alloys, from which it can be seen that there are remarkable differences between the quenching processes, as well as the cooling rates of the sites at different thicknesses of the products under certain conditions; in particular, the quenching rate of the core of the product is much lower than that of the surface. FIG. 2 shows the dimension and distribution of the second phase formed by the decomposition of supersaturated solid solution of alloys during the quenching, from which it can be seen that the supersaturated solid solution of alloys is decomposed due to the lower quenching rate around the core of the product, and a large amount of solute elements are precipitated and grown to relatively coarser quench-precipitated phase. The generation of such coarse quench-precipitated phase not only reduce the degree of supersaturation of the solute element within the matrix of the core of alloy product so as to reduce the amount of the precipitation-strengthened phase formed during the subsequent aging treatment and deteriorate the strength property at these sites, but also is likely to become the origin of crack initiation and micro-area corrosion so as to deteriorate other properties of the site, for example, elongation, fracture toughness, fatigue property, corrosion-resistance, and the like. Meanwhile, it can be also seen that solute elements are less or hardly precipitated in proximity of the surface of product due to the relatively higher quenching rate, and the supersaturation of the solute elements within the matrix, thereby facilitating the formation of adequate, fine, suitably distributed, precipitation-strengthened phase during the subsequent aging process, such that the desired good comprehensive performance of alloys can be maintained in proximity of the surface of products.
More intensive research results indicate that the affect of the quenching rate on the decomposition behavior of supersaturated solid solution of 7xxx series aluminum alloys are primarily due to two aspects as described below.
The first aspect is the so-called “stability of supersaturated solid solution”.
In 7xxx series aluminum alloys, it is well known that Zn, Mg, and Cu are primary alloying elements. The addition of Zn and Mg is mainly for the purpose of forming a precipitation-strengthened phase having a chemical constitution of MgZn2 and in a coherence relation with the matrix in the alloys. Furthermore, on the one hand, the addition of Cu is mainly for the purpose of improving the corrosion resistance of alloys by modifying the electrode potential of the alloys by solutionizing Cu in the matrix or the precipitated phase; and on the other hand, the presence of Cu can accelerate the formation of the precipitated phase and enhance the stability at an elevated temperature. When the level of Cu exceeds the solid solubility limit thereof in the matrix and the precipitated phase, a precipitation-strengthened phase having a chemical constitution of Al2Cu and other Cu-enriched ternary phase and quaternary phase can be formed and produce an additionally strengthening effect. For years, persons skilled in the art make an effort to enhance the strength, toughness and corrosion resistance of 7xxx series aluminum alloys; and up to now a full set of theories and methods for controlling the level range of primary alloying elements Zn, Mg, and Cu have been established, on the basis of which a series of 7xxx series aluminum alloys having various properties and characteristics have been developed. However, in recent years, it have been found that some alloys prepared at a certain ratio of three primary alloying elements Zn, Mg, and Cu within the level range of conventional 7xxx series aluminum alloys can form, during the quenching process subsequent to the solution heat treatment, a supersaturated solid solution exhibiting a good stability under slow cooling conditions, whereas alloys prepared with other ratios will form a supersaturated solid solution which is likely to be decomposed under slow cooling conditions. Summarized on the basis of observation, even though inherent microscopic mechanism is not completely known, it have been found that the stability of supersaturated solid solution under various cooling rate conditions are not sensitive to the change of Zn level in a relatively broad range, whereas are highly sensitive to the change of Cu level. In particular, excessive Cu is likely to cause a sharp falling of the stability of supersaturated solid solution of alloys under a certain quenching rate conditions.
The second aspect is the so-called “induced precipitation phenomenon”.
7xxx series aluminum alloys comprise inevitably impurity elements, such as, Fe, Si, or the like, and thus Fe-enriched phase, Si-enriched phase, etc. will be formed during the solidifying of alloys. Meanwhile, for the purpose of controlling the dimension of casting crystalline grains of the alloys and the growth of grains during homogenization and for inhibiting the occurrence of recrystallization during the thermal distortion processing and the solution heat treatment, a plurality of trace alloying elements (e.g., Ti, Cr, Mn, Zr, Sc, Hf, and the like) are added into the alloys to form some second fine phases capable of representing a pinning effect on the crystal boundary during solidifying of alloys, or precipitating some fine dispersed phases capable of both representing a pinning effect on the crystal boundary and contributing to a strengthening effect during homogenization of alloys. However, there are research results indicating that various second phases formed during solidifying of alloys, or even some dispersed phases precipitated during homogenization treatment of alloys are commonly in mismatching relation with the crystal lattice of the matrix, and thereby the second phases in mismatching relation with the lattice of matrix are likely to serve as the nuclei of “inducing” heterogeneous core of quench-precipitation phase. The micrographs as shown in FIG. 3 illustrate the preferential precipitation of the quench-precipitation phase at the sites of the aforesaid second phases in mismatching relation with the lattice of the matrix.
In recent years, the before described problems have caught extensive attention of many research institutions and companies. On the basis of abundant laboratory research in combination of theoretical calculation and analysis, a series of high performance 7xxx series aluminum alloy materials which exhibit good over-all properties and are relatively less affected in terms of various properties by the change of product thickness (i.e., the so-called “low quench sensitivity”) have been developed by optimizing the components of alloys, combined with optimizing the preparing, molding, and heat-treating processes thereof.
For instance, (1) CN1489637A, which is submitted by Alcoa Inc. (a U.S. company) and published in 2004, discloses a low quench sensitivity, high-strength and high-toughness aluminum alloy adapted for the production of large thickness structural components, consisting essentially of: Zn 6-10 wt %, Mg 1.2-1.9 wt %, Cu 1.2-1.9 wt %, Zr≤0.4 wt %, Sc≤0.4 wt %, Hf≤0.3 wt %, Ti≤0.06 wt %, Ca≤0.03 wt %, Sr≤0.03 wt %, Be≤0.002 wt %, Mn≤0.3 wt %, Fe≤0.25 wt %, Si≤0.25 wt %, and balance Al. Also, the aluminum alloy preferably comprises Zn 6.4-9.5 wt %, Mg 1.3-1.7 wt %, Cu 1.3-1.9 wt %, Zr 0.05-0.2 wt %, wherein Mg wt %≤(Cu wt %+0.3 wt %). As listed in the embodiments of CN1489637A, under T7 “over-aged” conditions, the yield strength/fracture toughness in the longitudinal (L-) direction of the core of a plate product made from typical alloys may be up to 516 MPa/36.6 MPa·m1/2 when the plate product has a thickness of up to 152 mm; and the process of heat treatment may be adjusted to increase the yield strength and decrease the fracture toughness, or to decrease the yield strength and increase the fracture toughness. Moreover, the yield strength of the core of products may be up to 489 MPa (in the L-direction)/486 MPa (in the LT-direction) when the forging piece made from typical alloys have a thickness of 178 mm. In that case, the products may exhibit much better elongation, fatigue, as well stress corrosion resistance and exfoliation corrosion properties, compared with those having a similarly greater thickness and made from conventional alloys 7050, 7150, 7055, and the like, and exhibit a superior balance of various properties and low quench sensitivity.
(2) CN1780926A, which is submitted by Corus Aluminum Walzprod GmbH (a German company) and published on 2006, discloses a high-strength and high-toughness aluminum alloy having superior balance of various properties, consisting essentially of: Zn 6.5-9.5 wt %, Mg 1.2-2.2 wt %, Cu 1.0-1.9 wt %, Zr≤0.5 wt %, Sc≤0.7 wt %, Cr≤0.4 wt %, Hf≤0.3 wt %, Ti≤0.4 wt %, V≤0.4 wt %, Mn≤0.8 wt %, Fe≤0.3 wt %, Si≤0.2 wt %, other impurities or incidental elements each ≤0.05 wt %, total ≤0.15 wt %, and balance Al; preferably, (0.9 Mg-0.6)≤Cu≤(0.9 Mg+0.05). As listed in the embodiments of CN1780926A, under T7 “over-aged” conditions (including T76 and T74), the ultimate tensile strength/yield strength/elongation/fracture toughness/exfoliation corrosion properties at the site of ¼ thickness of the products can be up to 523 MPa/494 MPa/10.5%/39 MPa·m1/2/EA, when the plate products made from typical alloys have a thickness of up to 150 mm, and the process of heat treatment may be adjusted to increase the yield strength and decrease the elongation and fracture toughness, or to decrease the yield strength and increase the elongation and fracture toughness. In that case, the products may exhibit a superior balance of various properties and a low quench sensitivity.
(3) Similar works have been also reported in other publications.
Although the aforesaid attempts have achieved some success, there is a continuing requirement of large thickness products of 7xxx aluminum alloys having better over-all properties and exhibiting more homogeneous properties inside of the products with the rapid development of modern aerial manufacturing and other relevant technologies. Thus, persons of skill in the art do not draw in the reins in this regard. Surprisingly, 7xxx series aluminum alloys would satisfy the aforesaid rigorous requirements if the content range of each component and the percentage of each element thereof are optimized more carefully.